Why do tires slowly lose air? From what it seems, they are sealed and shouldn’t really be letting anything in or out, but they still need to be refilled every now and then.

Question

I understand pressure changes as heat changes, but even in the same season you will still lose a slight amount of pressure.

In: 141

Answer 1

When you think on a microscopic level, the elastomers in the tire actually have holes between the molecules — large enough to let some air molecules through.

Balloons are a great illustration of this. A balloon full of air will “shrink” slower than one full of Helium, which has a much smaller molecule.

Decades ago, for an outdoor event I was at, bought a tank of hydrogen to fill balloons. It worked great — but leaked through the rubber fairly quickly. Half an hour after inflation, they wouldn’t hold themselves up.

A side note. A “Helium Leak Test” is often performed on critical metal instrumentation/components. If there are faulty welds, the helium will leak through the tiniest of spaces and is easily detected.